Perimeter intrusion detection system



April 16, 1968 P. I. CORBELL PERIMETER INTRUSION DETECTION SYSTEM FiledMay 10, 1966 3 Sheets-Sheet 1 4 I I DETECTOR 7 III|IIIIIIIIIIIIIIIIIIIIIIIIIIII.

LOW PASS FILTER ALARM TEMPERATURE OOMPENSATOR I l I I l l I I I l I l lI I I l I l J- DEMODULATOR HIGH PASS FILTER SUPERVISOR Z R-F 6 SOURCESUPPLY CONVERTER 0.0 VOLTAGE INVENTOR. PAUL 1 (meta Jna s Sim-1V2Jf/a/I'e 5 April 16, 1968 P. CORBELL PERIMETER INTRUSION DETECTIONSYSTEM 3 Sheets-Sheet 2 Filed May 10, 1966 INVENTOR. 840; I (aRBe-LLAffa/we s April 16, 1968 P. I. CORBELL 3,378,834

.PERIMETER INTRUSION DETECTION SYSTEM Filed May 10, 1966 5 Sheets-Sheet25 DC TO DC CONVERTER C 4 A DETECTOR 9/ D7LAY AND A; w

7; 77 5/ AGC CIRCUIT Q SQUARE wAvE AMPLITUDE SIGNAL 0 DETECTORSUPERK'SORY ALARM CIRCUIT ALARM x I NVENTOR v PAUL CoRBHL BY dyndrus IJimflffagyys United States Patent 3,378,834 PERIMETER INTRUSIONDETECTION SYSTEM Paul I. Corbell, Milwaukee, Wis, assignor to .lohnsonService Company, Milwaukee, Wis, a corporation of Wisconsin Filed May10, 1966, Ser. No. 548,983

- 17 Claims. (Cl. 343-) ABSTRACT OF THE DISCLOSURE A perimeter typecontrol controls entrance to an area and includes a transmitter at oneend and a receiver at the opposite end. A beam of high or radiofrequencyenergy of the order of 200 megacycles or higher is established. Thereceiver, such as a radiofrequency detector, is fed into a high gainamplifier the output of which is fed through a temperature stabilizingcircuit. An alarm circuit and a supervisory circuit are parallelconnected to the amplifier. The alarm circuit is selected such that itwill not detect the change in the Doppler frequency signal establishedby slow movement within the beam. The supervisory circuit howeverdetects the continued proper operation of the circuit and the state ofthe carrier signal. Slow moving targets substantially decrease thecarrier signal which in turn will trigger the alarm and indicate themalfunctioning of the circuit and/or the presence of a slow movingtarget. Further the amplifier section is provided with a feedbackcircuit including an amplitude detector and a delayed automatic gaincontrol circuit. The output of the amplifying section can be fed into atiming capacitor to introduce a preselected time delay in the transferof a proportional signal which will be fed into a control transistorhaving a variable impedance to vary the gain of the control transistorand thereby the amplifying section.

This invention relates to a perimeter intrusion detection system andparticularly to such a system for producing an output signal bydetecting the presence of the intruder.

Detection of intrusion into restricted areas has suggested the use ofvarious forms of radiated energy into the area with the intruding personor object causing a change in the energy distribution. This change inenergy distribution is sensed and caused to actuate a suitable alarm orthe like. A highly sensitive and reliable intrusion detection system isdisclosed in applicants copending application entitled, IntrusionDetection System, which was filed on Apr. 20, 1962, with Ser. No,189,170, now US. Patent 3,242,- 486, and assigned to a common assigneewherein a radio frequency energy field is established and the creationof a Doppler frequency as a result of movement within the field providesa means for detecting movement of foreign bodies. The system disclosedtherein particularly suggests the use of the reflected energy from thebody to a receiver where it is mixed with a portion of transmittedenergy which is also fed to the receiver. That system in contrast to theprior art integrating system employs an amplitude discriminating meanssuch that it will respond to a single cycle of a Doppler signal of aselected magnitude. Although such system has been found to produceunusually satisfactory results for indoor detection or for limitedoutdoor areas, the sensitivity is such that when employed to cover largeoutdoor areas, the generation of spurious alarm signals presents asevere problem. Thus, as the area to be protected is extended, thetarget distance from the receiver increases. In order to make the systemsensitive to movement at the outer extremes, the transmitted energylevel must the substantially increased or the receiver must operate at afairly high level of sensitivity. As a result, the energy immediatelyadjacent the transmitter and the receiver increases to such level thatprecipitation, blowing "ice paper, leaves, small animals and the likehave a tendency to cause a triggering of the alarm. If the sensitivityis reduced such that such spurious alarm signals do not occur, themotion of an object in the outer periphery area of the field will not bedetected.

Although capacitance, sonic and ultrasonic field systems have also beensuggested for outdoor protection, such systems are not sufficientlytolerant of wind, environmental noise and the like and thus suffer fromsimilar disadvantages as those described above. Beamed energy betweenreceivers and transmitters using relatively low frequencies, generallybelow 2000 megacycles have also been suggested. Generally, the frequencyis held low because higher frequencies generally detect small objects aswell as rain and other forms of precipitation and consequently have beenconsidered undesirable. Such systems should be provided with highsensitivity in order to provide effective motion detection.

The present invention is particularly directed to a perimeter typedetection unit wherein a transmitter and receiver are positioned atopposite ends of a perimeter line to provide a beam of high or radiofrequency energy which is selected to be what would normally beconsidered of an undesirably high frequency; that is, in the order of2000 megacycles or higher. This system employs highly directional andrelatively narrow beams with a crystal receiver or the like which has arelatively low sensitivity, The output circuit is designed to respond tothe Doppler effect and the high transmitted frequency provides asufficient diiference in the motion detection signal for convenientsignal manipulation. The high frequency signal is modulated either bymodulating the transmitted signal or the received signals. The output ofthe receiver or R-F detector is followed by a very high gainamplification at the 1 modulating frequency. This is then demodulatedand divided into a Doppler difierence signal which is extracted from themotion detected signal and a fail-safe signal.

The system may be adjusted such that very slow motions will not (bedetected; that is, maximum motionsensing capability is not necessarilyemployed to detect a moving target. Rather, the present invention relieson detection of the carrier signal as a part of a fail-safe circuit totrigger an alarm for exceedingly slow targets. The failsafe signal levelmay be amplitude detected to either side of a normal level. Generally,however, it has been found desirable to detect a decrease in signallevel of 20% or more. If an intruder were to move very slowly throughthe energy beam, the motion detecting means may not receive a sufiicientsignal to trigger the alarm in the normal motion detecting channel.However, there would be a substantial loss in the carrier signal andconsequently the failsafe circuit would be triggered. Thus, in thepresent invention, the fail-safe circuitry performs the dual functionsof fail-safe protection and motion intrusion detection, especiallyextremely slow motion. This has the distinct advantage of eliminatingthe many problems inherent in previously suggested solutions such asincreasing of power levels, circuit gains and sensitivity and the like.

A further problem encountered in motion detection, particularly whereextremely slow motion can be detected, is false triggering as a resultof slowly changing environmental conditions such as accumulations ofsnow, debris and the like. In accordance with another aspect of thepresent invention, the receiver includes means to modify the detectedsisignal to eliminate the efiects of extremely slowly changingconditions without adversely reducing the sensitivity to human movementwithin the field. Thus, the receiver normally includes an amplifyingsection which is provided with a feedback circuit including an amplitudedetector and delayed automatic gain control ircuit. The control circuitmay include a control transistor as a variable impedance elementconnected in the bias circuit 3 of an input stage of the amplifyingsection. The control transistor is in turn biased by the output of theamplifying section with a timing capacitor introducing a preselecteddelay in change of the gain of the control transistor and therefore theamplifying section.

The present invention provides a highly improved perimcter typeprotection system which can be employed in outdoor environments withoutthe usual problems encountered by spurious generating signal elements.

The drawings furnished herewith illustrate one mode of carrying out thepresent invention clearly disclosing the above advantages and featuresas well as others which will be clear from the following description.

In the drawings:

FIG. 1 is a block diagram illustrating the perimeter control intrusiondetection system constructed in accordance with the present invention;

FIG. 2 is a schematic circuit diagram of a suitable transmitter unit;

FIG. 3 is a schematic circuit diagram of a suitable receiver unit.

FIG. 4 is a block diagram of a modified system; and

FIG. 5 is a schematic circuit diagram of FIG. 4.

Referring to the drawings and particularly to FIG. 1, the radiofrequency source or transmitter 1 includes an antenna 2 which is adaptedto establish and transmit a high or radio frequency energy beampreferably having a frequency of the order of 10,525 megacycles. Inaccordance with the present invention, the radio frequency or thetransmitter 1 creates a pulsed radio frequency energy beam showndiagrammatically in FIG. 1 by the pulse lines 3. The energy field is ahighly directional and relatively narrow beam and is directed to analigned receiving antenna 4 of a radio frequency detector 5 such as theknown crystal detector device to transmit the received energy and properamplification and demodulation to a motion detection channel 6 and afail-safe channel 7, although for crystal detectors and the like, thislimits the level of detection and it has generally been found that alength of 100 feet can be effectively detected with the receiver locatedas high as five or six feet above the ground level.

Although any suitable components can be employed, the transmitter orsource 1 includes a D.C. (direct current) supply 8 which is connected toa D.C. to D.C. converter to provide D.C. voltages suitable for operationof transmitter 1 which is modulated to provide the desired modulating orpulse frequency. The receiving system includes a pro-amplifier 10connected to the output of the detector 5 and selected to provide veryhigh gain at the modulating frequency. In the illustrated embodiment ofthe invention, a temperature compensator circuit 11 is connected tomodify the amplified signal and compensate for the wide variation intemperatures which will be encountered in many outdoor environmentalinstallations. In certain climates for example in the northern part ofthe United States, temperatures may vary from about 30 to about 120. Theresponse of the various components of the system particularly employingsolid state units which are required for iugged and reliable operationcause the system to be temperature sensitive.

The compensated signal is impressed upon the channels 6 and 7 inparallel. Channel s includes a demodulator 12 to remove the modulatingfrequency and a low pass filter 13 which passes only the Dopplercomponents. The filtered Doppler components are again amplified by anamplifier 14 to a suitable operating level for triggering a motion alarmcircuit 15 or other suitable output.

The fail-safe channel 7 similarly includes a demodulator 16 and a highpass filter 17 to remove the low frequency Doppler signals to thetransmitter by a relatively steady D.C. signal component which isamplified by an amplifier 18 and fed to a supervisory and alarm circuit19.

More particularly, the component shown in block diagram may take theform of the circuitry shown in FIGS. 2 and 3 which respectively show thetransmitters and in schematic form. particularly to FIG. 2, theillustrated transincl' cs a D.C. to D.C. converter circuit .r ofswitchin transistors 20 connected to the D.C. supply 8 and coupled by atransformer 21 to a low voltage D.C. filament power circuit 22 for aklystron tube 23 having a filament 24 connected to circuit 22. Tube 23is a well znown device for generating R-F energy and includes areflector 25, beam grids 26 and a cathode 27. A. rectifier is separatelyconnected to the output of the D.C. to D.C. converter transformer 21 ina three wire output having a common center tap or ground line 29, a 250-volt positive line and a 50- to 200-volt negative line A filter circuit32 is connected to the output side of the D.C. supply between lines 38and 31 to provide a relatively constant D.C. voltage which is applied asthe beam voItage between grids 26 and cathode 27 of the klystron tube23. The reflector voltage of the klystron tube 23 is taken between lines29 and 30 which includes a specially selected poor filter 33 whichmaintains a D.C. output having a selected ripple component superimposedthereon. In the illustrated embodiment, filter 33 includes a capacitor34 connected between the D.C. lines 29 and 31 with a resistor 35connected in series in line 29. A pair of voltage dividing resistors 3-6and 37 is connected in parallel with capacitor 34 and resistor 35between lines 29 and 31 to complete the filter circuit. A potentiometer38 is connected across the resistor 37 and includes a potentiometer tap39 connected to reflector 25. Potentiometer 38 may be adapted to varythe reflector voltage between 50 to 200 volts. The filter circuit 33 maybe selected to maintain a substantial A.C. type modulation generally attwo times the switching rate of the converter circuit 20-. Thus, thecircuit may operate at a switching rate of Z lcilocycles (kc.) andproduce a ripple frequency of 4- kc. The reflector voltage is adjustedto cause the klystron tube 23 to switch in and out of oscillation andthereby provide modulation of the transmitted signal. This provides avery convenient and inexpensive manner of providing the pulsed radiofrequency signal and field between the transmitting unit 1 and thereceiving unit 5.

Other reams of modulation and other means of generating a radiofrequency may of course be employed and, as more fully describedhereinafter with respect to FIG. 4, it is sometimes highly desirable topositively modulate the reflector volta e by a superimposed square wavemodulating signal or the like. Thus, the output signal might bemodulated employing a mechanical flapper or an electronic switchoperating at the microwave level.

At the receiving end, the antenna 4 impresses the transmitted energyupon a crystal detector 5 or the like, the output of which is connectedto the high gain pro-amplifier 10. K

The pre-amplifier 19 is preferably a multiple stage unit. Theillustrated circuit is a well known unit employing a first stagetransistor 40 connected in a common emitter configuration and with aboot-strap type feedback connection. A capacitor and resistor .1 connectthe emitter-base circuit to provide the desired feedback for aboot-strap circuit. The multiple stage pro-amplifier 10 with theboot-strap construction is highly desirable in that such circuits have ahigh input impedance and permits ready cascading thereof without loadingof succeeding stages. This provides a very simple means of providingvery high gain for example in the order of 200,000.

The amplified output is fed to temperature compensating circuit 11 whichin the illustrated embodiment of the invention includes a voltagedividing branch including an input resistor 42 and a temperaturecompensating thermistor 43. The thermistor 4-3 has a substantiallygreater resistance when cold than when hot. The output of the simpledivider network supplies an output voltage which is maximum at coldtemperatures and minimum at hot temperatures. This will provide somecompensation for the temperature characteristics of the usual solidstate amplifier 10 and the crystal detector 5 forming the immediatelypreceding stages of the receiving unit. The illustrated compensatingcircuit 11 includes amplification means including an isolatingemitter-follower transistor 44, an unbypassed emitter-resistor amplifiertransistor 45 and an output emitter follower transistor 46. Transistors44 and 46 isolate the signal from the adjacent sections and transistor45 is unbypassed to aid in its temperature stabilization. The output ofthe emitter-follower 46 is capacitive coupled to channels 6 and 7.

Referring particularly to channel 6, the illustrated demodulatorincludes a diode 47 connected to the emitterfollower 46 by a variablepotentiometer 48 having an adjustable output tap 49. The setting of thepotentiometer tap 49 controls the operative incoming signal level foramplitude discrimination as hereinafter described. The output of thedemodulator 12 is filtered by the low pass filter 13 including acapacitor 50 and a resistor 51 connected in parallel with the signaltransmitting circuit. The demodulator in conjunction with the associatedfilter circuit removes the four kilocycle carrier and passes only thelow frequency Doppler component which is generated by movement withinthe beamed field. This Doppler signal is fed to amplifier 14 whichincludes a pair of transistors 52 and 53. Transistor 52 is shown as abootstrap unit having high input impedance. Another advantage of thiscircuit is that the high input impedance permits the use of moderatesizes of electrolytic capacitors in the amplifying circuit. A capacitor54 is connected between the collector of transistor 52 and ground toremove any high frequency noise which might interfere with the desiredoperation of the circuit. Transistor 53 is connected as an emitterfollower to avoid loading of the motion detector or alarm circuit 15.

Circuit 15 may be of any desirable form but is preferably similar tothat disclosed in applicants previously referred to patent and includesa Schmitt trigger circuit 55 coupled to energize an alarm relay 56 orother suitable detection means. As more fully described in the abovepatent, the illustrated Schmitt trigger circuit includes a pair oftransistors 57 and 58 having an initial stable state wherein transistor57 is conducting and transistor 58 is quiescent or cut off. When theincoming input signal applied to the transistor 57 rises above thethreshold level, it will cause a reversal in the state of thetransistors 57 and 58. When such signal drops below the threshold level,the circuit will reverse to its standby or initial starting position.Relay 56 has a capacitor 59 paralleled with its winding 60 and connectedin parallel with the output of the transistor 58 and consequently isenergized when the transistor 58 is cut E and is deenergized when thetransistor :58 is turned on. The relay 56 is spring loaded or otherwisebiased to an alarm position and it is held in an off or non-alarmposition by energization. This is desirable to provide the normalfail-safe operation such that in the event of failure of the motiondetection channel the relay will be released and move to the alarmposition to indicate the fact that the intrusion system is notoperative.

Addiionally, in the il lustratedcircuit, a diode 61 is paralleled with aresistor 62 between relay 56 and the collector of transistor '58. Thisdecreases the reset time of the relay circuit.

The operation of the marized as follows.

If a moving body is present in the high energy beamed field betweenantennas 2 and 4, the alternating current signal detected by thedetector includes a Doppler frequency. The Doppler modulated signalamplified by amplifier and compensated by circuit 11 reduces the effectsof temperature variation. The amplified signal is then applied tochannels 6 and 7. The potentiometer 48 of circuit 6 transmits a selectedproportion of the sigdetection system is briefly sumnal to filter 13which removes high frequencies components and transmits the Dopplerfrequency component to amplifier 14. The amplified motion-related orDoppler signal is applied to the Schmitt trigger circuit and if ofsuificient amplitude, causes the circuit to reverse its state whereintransistor 58 is turned on. As a result, relay 56 is de-energized. Therelay 56 may control an alarm system having. means, not shown, tomaintain the alarm indication after the trigger signal is removed at theend of the cycle and the relay 56 reverts to the normal energizedposition. The amplitude of the Doppler signal for any given object willbe essentially independent of the position of the object between thetransmitter 1 and the detector 5. For example, if the object isimmediately adjacent the transmitter 1, the energy intercepted will beat or near a maximum. However, the Doppler-shifted signal will have tomove through essentially the complete length of the path beingprotected. Consequently, the losses reduce the signal to a selectedlevel. On the other hand, if the intrusion is made adjacent the detector5, the intercepted energy is of course relatively low as a result of thelosses in energy transmission. However, the Doppler-shifted signaltravels a very short distance to the detector 5 and is received thereatat substantially the same level as in the initial case wherein themoving object was adjacent the transmitter. In summary, the level of theintercepted signal varies inversely with the distance from thetransmitter 1. The speed or rate of movement of the that the Dopplersignal travels a distance which similarly varies inversely with thedistance between the body and the transmitter 1. The speed of rate ofmovement of the target however directly effects the frequency and thedetermination of the Doppler signal and the duration that the Dopplersignal is present. In accordance with the present invention, the motiondetection channel 6 may not detect exceedingly slow motion of even largebodies such as a human being. To design the system with sufiicientpower, gain and sensitivity creates corresponding sensitivity tounwanted spurious signals from precipitation, small objects and thelike.

The present invention employs the dual channels 6 and 7 with thesupervisory channel 7 also serving to detect the exceedingly slow motionof a large body.

The amplified and temperature compensated signal from the compensator 11is coupled to channel 7 by a small capacitor 63 in series with apotentiometer 64 having a tap 65. A diode 66 connects tap 65 to theamplifier 18 with the high pass filter 17 connected therein. Capacitor63 is a small unit which essentially blocks the low frequency Dopplermodulation such that only the subcarrier frequency signal created by the4 kc. pulsing of the energy field appears in channel 7. The filter 17 isa capacitor-resistor unit which further insures removal of anyextraneous signals and provides a relatively steady D.C. componentcorresponding to the level of the incoming carrier signal. The rectifiedcarrier signal is amplified by amplifier 18 which is shown as a singlestage transistor 67. The amplified signal is fed to a low level Schmitttrigger circuit 68 and forming a part of supervisory circuit 19. Thus,circuit 68 includes a pair of transistors 69 and 70. Transistor 69 isnormally biased on or conducting by the incoming signal from transistor67 and the transistor 70 is correspondingly biased off. A supervisoryrelay 71 is connected in shunt with the output circuit of the transistor70 between the collector of the latter and the positive side of thepower source, as shown. Consequently, relay 71 is normally energized andholds an alarm in the off position. If the modulating signal is lost forany reason, the transistors 67 and 69 will be cut off. When transistor69 stops conduction, the other transistor 70 of the Schmitt circuitconducts. Transistor 70 then provides a very low resistance and aneffective short circuit in parallel with relay 71 which is thendeenergized and causes an alarm signal.

Although not absolutely necessary, a resistor 72 is shown in theconnection of the emitters of the transistors 69 and 75B of the lowlevel circuit 68. This increases the capability of discriminating inputlevels at very exceedingly low voltages.

The supervisory channel 7 is responsive to the presence of themodulating frequency signal and i s absence of a decrease below aselected level triggers the circuit 19. As previously described, thepresent invention employs the channel '7 to supervise the transmissionof the radiofrcqucncy energy and further to detect ver slow movingtargets. Thus, a very slow moving target of a size of a human or thelike passing through the radio-frequency field results in a substantialreduction in the carrier signal transmitted to the receiving unit andtherefore to channel 7 which will cause de-energization of relay 71.This is of substantial significance in eliminating the problems inherentin trying to adjust a system to otherwise detect all motion in a fieldand yet not detecting spurious signals generated by the surroundingenvironment and small objects moving through the field.

In the illustrated embodiment of the invention, the crystal detectoremployed may be energized with a slight direct current bias. Thisincreases its sensitivity somewhat but more importantly tends to provideincreased temperature stabilization.

Although the systems shown above provide highly satisfactory operation,it had been found that the slow alarm system may be affected andactuated by slowly changing normal conditions; for example, atmosphericchanges, accumulations of debris, snow and the like. Further, when aplurality of the free running transmitting devices are employed instacked relation, in order to increase the protective area, cross-talkeffects may be introduced and cause malfunctioning.

Another embodiment of the present invention is shown in FIGS. 4 andwherein the above disadvantages are essentially eliminated.

Referring particularly to FIG. 4, the system is shown generally in blockdiagram with elements corresponding to those shown in FIG. 1 similarlynumbered for simplicity and clarity of explanation. In FIG. 4, amodified transmitting system is shown particularly adapted for multiplestacked unit detection systems. Thus, in FIG. 4, the power supply forthe klystron transmitter 2 is generally similar to that previouslydescribed with respect to FIGS. 1 and 2. In the embodiment of FIG. 4,however, the reflector supply is provided with a filtering stage toestablish an essentially ripple free DC. signal at the tap 39 and thuswithout the ripple component employed in the embodiment of FIG. 2.Superimposed on this DC. signal is a square wave signal to provide thedesired pulsing operation of the transmitter 2 by control of thereflector voltage. In the illustrated embodiment of the invention, asquare wave signal source 75 is shown in block diagram and coupled intothe reflector voltage lead by a suitable transformer 76. A secondtransformer '77 has its primary connected in parallel with the primaryof the transformer 76 and thus provides a source to a second klystrontransmitter 78. The power supply and receiver for transmitter 78 is notshown. 1

The transmitter of FIG. 4 operates in the same general manner as thatpreviously described except that a synchronized pulsing of the energy isprovided and maintained without adverse interaction between the twosystems.

The receiving circuitry is also generally similar to that previouslydescribed and includes a suitable detector 5 having an amplifier foramplifying the detected signal and applying the signal simultaneously tothe supervisory and alarm circuit 7 and to the alarm circuit 6. Inaddition, in the embodiment of FIG. 4, the amplifier It) is providedwith a feedback system 79 to modify the output signal in a mannercompensating for exceedingly slow amplitude changes in the receivedsignal such that the circuit '7 is not triggered by very slow changingenvironmental conditions such as caused by changes in the weather,accumulating debris and the like. Generally, the feedback system '79includes an amplitude detector 8:) and a time delay and automatic gaincontrol unit 81 interconnecting the output of the amplifier 10 to aninput stage, as more fully shown in FIG. 5.

In operation, the system responds to both slow and rapid movement withinthe field to trigger the supervisory and alarm circuit 7 or alarmcircuit 6. However, the feedback systcm 79 established by the amplitudedetector 80 and the delayed automatic ain control unit 81 providesautomatic compensation for exceedingly slow amplitude variations in thereceived signal such that they cannot trigger the supervisory and alarmcircuit 7.

More particularly, a preferred construction of the feedback system 79and its connection is shown in detail in FIG. 5 along with modificationof the supervisory and alarm circuit.

In FIG. 5, the amplifier Itl is shown having a modified input stageincluding a transistor 82 of an NPN variety connected in a commonemitter configuration. The base of the transistor 82 is connected by asuitable coupling network to the output of the detector. Thecollectoremitter circuit is connected to suitable positive and negativepower supply lines with the collector connected in the conventionalmanner. The emitter 83 of transistor 82 is connected to the negativeline 84 through a paralleled resistor 85 and a capacitor 86 and aportion of the delay and automatic gain control circuit 81. The feedbacksystem 79 modifies the emitter bias on the transistor 82 to compensatefor slow variations in the changes in the output signal such as would becaused by changing weather conditions, accumulating debris and the like.The amplitude detector 80 of system '79 is shown including a largecoupling capacitor 87 coupled to a diode detector circuit including adiode 88 in series with a Zener diode 89. A resistor-capacitor network90 is connected to the output side of the Zener diode 89 to provide thedesired high degree of rectification efiiciency and provide a directcurrent signal which is amplified by an amplifying transistor 91. Theamplified signal is connected through a coupling circuit 92 to the timedelay and automatic gain control circuit 81.

In the illustrated embodiment of the invention, the circuit 81 includesan emitter-follower transistor 93 as the input stage. A transistor 94 isconnected to transistor 93 to amplify the follower signal and to biasthe emitterfollower output transistor 95. The transistors 93, 94 and 95are shown as NPN type transistors and the emitter 96 of the followertransistor 95 is connected by a large coupling capacitor 97 to the base98 of the first stage transistor 93. The follower is also connected by aload and coupling network 99 to a control transistor 100.

The control transistor 100 has its emitter to collector circuitconnected in series between the paralleled resistor 85 and capacitor 36of the amplifier 10 and the negative signal power supply line 84. Thetransistor 100 constitutes a variable impedance element connected in thecircuit of the emitter 83 of the transistor 82. This in turn controlsthe conductivity of the transistor 82 to compensate for slow movingsignals as follows. The capacitor 97 provides a feedback path from thetransistor 95 to the input of transistor 93. The capacitor 97 isrelatively large and consequently prevents rapid changes in the inputfrom affecting the output signal. However, over long periods of time, acharge on the capacitor 97 is established which effects the amplifyingoutput of the circuit and thereby varies the input bias on the variableimpedance transistor 100. The time constant is selected to preventtriggering of circuit 7 under changes occurring over relative longperiods while maintaining the system sensitivity to unauthorizedpersonnel movement which even though extremely slow will not approachthat of the com- 9 pensated changes. As a practical matter, it has beenfound that a time constant of approximately five minutes or so providesa highly satisfactory control circuitry.

In summary, the output of the amplifier 1G is continuously detected toenergize the delay and automatic gain control circuit 81 which inserts aselected time delay into the response modification such that it willrespond to only changes occurring over a very long period of time. Inthis manner, the response modification compensates for and effectivelyeliminates signals only for changes in the environment of the beam overperiods substantially greater than the selected minimum motion of atarget within the beam which shall actuate circuit 7.

The circuits 6 and 7 can be constructed in the same manner as that shownin the previous embodiment. In FIG. 5, an alternative embodiment of thesupervisory and alarm circuit 7 is shown. In FIG. 5, the output signalof the amplifier 10- is connected through a suitable low pass filtercircuit 1-01 to a transistor amplifier including an input biastransistor 102 and an output stage transistor 103. The relay 71 isconnected in the collector circuit of the transistor 103. This circuitemploys a bias emitter stage rather than a regenerative Schmitt triggercircuit such as shown in FIG. 3. The circuit of FIG. has been operatedsatisfactorily.

The transmitted signal may also be modulated either at the transmitteror the receiver employing a mechanical flapper or with an electronicswitch.

Further, in the illustrated embodiment of the invention, the transmittedsignal is shown as the modulated signal. Where extreme security isrequired, the pulse rate signal can be transmitted separately to thereceiving station for synchronous demodulation. This will also improvethe signal to noise characteristics. However, the added complexity andinitial cost of the receiving system generally limits such applicationshaving high security requirements such that cost considerations aregenerally secondary.

The present invention thus provides a perimeter type alarm system whichcan be employed in the outdoor environment without generating spurioussignals as a result of small targets such as paper, leaves, smallanimals, precipitation and the like.

Various modes of carrying out the invention are con templated as beingwithin the scope of the following claims particularly pointing out anddistinctly claiming the subject matter which is regarded as theinvention.

I claim:

1. In a perimeter type intrusion detection system for detecting themovement of a traget of a preselected minimum size,

means for radiating a directional beam of energy having a selectedfrequency characteristic, detection means mounted in spaced alignedrelation to intercept said beam of energy and including response meansto detect changes in the frequency characteristic in response tomovement of a target and being constructed and arranged to require apredetermined minimum speed, such target movement within the beamchanging the frequency characteristic and producing a Doppler frequencysignal,

an output circuit means connected to the detection system and beingresponsive to the Doppler frequency signal and actuated by essentially asingle cycle of said Doppler frequency signal of a selected minimumfrequency and amplitude, said output circuit means including a signalamplifying section connected to the detection means, and a feedbackcircuit in the section and including a signal delay means, said feedbackcircuit modifying the amplifying section to compensate for changes inthe environment of the beam over periods substantially greater than theselected minimum motion of a target within the beam.

2. The intrusion detection system of claim 1 wherein said amplifyingsections include an input transistor connected in a common emitterconfiguration, a control transistor, said input transistor having anemitter connected to a supply means in series with said controltransistor, and said feedback circuit being connected to bias thecontrol transistor and including a time delay means.

3. The intrusion detection system of claim 1 wherein said feedbackcircuit includes an amplitude detector connected to the output of theamplifying section and a time delay automatic gain circuit connectingthe amplitude detector to the input of the amplifying section.

4. In an intrusion detection system for detecting the movement of atarget of a preselected minimum size,

source means for radiating a beam of energy having a selected frequencycharacteristic,

detection means mounted in spaced aligned relation to intercept saidbeam energy and including means to detect changes in the frequencycharacteristic movement of a target within the beam being effective tochange the frequency characteristic to produce a Doppler frequencysignal,

means to modulate the energy to provide a selected modulationcharacteristic in the detection means, and

a pair of parallel output circuits connected to the detection system,one of the output circuits being responsive to the Doppler frequencysignal and the second output circuit being responsive to a selectedchange in the signal from the detection means.

5. The detection system of claim 4 wherein the second output circuit isresponsive to loss of a signal of the modulation characteristic.

6. The detection system of claim 4 wherein means to modulate the energyforms a part of the means for radiat ing a beam of energy.

7. The detection system of claim 6 wherein said means to modulate is apulsing means to create a pulsed modulated beam.

8. The intrusion detection system of claim 4 wherein said pair ofparallel output circuits is respectively responsive to changes in thefrequency characteristic and to signal changes of the modulationcharacteristic, said first output circuit being selected to respond tofrequency characteristics generated by targets of a preselected size andrate of movement and said second output circuit being selected torespond to a selected loss of the signal of the modulationcharacteristic.

9. An intrusion detection system for detecting the movement of a body ofa preselected minimum size, comprising source means to generate a radiofrequency and adapted to be mounted adjacent one end of an area and todirect the radio frequency signal as a beam across an entrance to saidarea,

a detector adapted to be mounted in spaced alignment with the sourcemeans to intercept the beam at the opposite end of the entrance to saidarea,

the energy in said beam being essentially inversely proportional todistance from the source whereby the detector is energized at a similarlevel for any given target within the entrance,

a first demodulating channel connected to the detector to produce asignal corresponding essentially only to the Doppler components of theintercepted signal, and

a second demodulating channel connected to the detector to produce asignal corresponding essentially to only the carrier component of theintercepted signal.

10. The detection system of claim 9 having temperature compensatingmeans connected to the output of the detector to modify the signal andcompensate for environmental temperature variations.

11. The intrusion detection system of claim 9 wherein the first channelincludes a diode detector connected to an adjustable potentiometer forpresetting the minimum amplitude of the operative Doppler component.

l l 12. The intrusion detection system of claim 9 wherein the secondchannel includes a trigger circuit having a normally nonconductingtrigger element and responsive to the presence of a signal from thedetector to turn the trigger element on.

13. The intrusion detection system of claim 12 wherein the triggercircuit is a Schmitt trigger circuit.

14. The intrusion detection system of claim 13 having a transistor meansconnected as a variable impedance in the amplifying section to controlthe gain of the amplifying section and wherein said gain control circuitis connected to the transistor means and includes an amplifying meansincluding a timing capacitor.

15. The intrusion detection system of claim 9 wherein an amplifyingsection connects the detector to said channels, said amplifying sectionincluding a feedback circuit including an amplitude detector and anautomatic gain control circuit, said control circuit including a signaldelay means whereby only changes in the environment of the beam overperiods substantially greater than selected minimum motion of a targetWithin the beam.

16. An intrusion detection system for detecting the movement of a bodyof a preselected minimum size, comprising a klystron tube having a D.C.to D.C. converter connected to create a beam voltage and a reflectorvoltage, said converter selected to have a substantial ripple componentin the reflector voltage to provide a pulsed radio frequency signal,

said tube being adapted to be mounted adjacent one end of an area and todirect the pulsed radio frequency signal as a beam across the area,motion of a target within the area generating a Doppler modulation ofthe signal,

a crystal detector,

said crystal detector being adapted to be disposed in spaced alignmentwith the klystron tube to intercept the beam at the opposite end of thearea, the energy in said beam being essei ially inversely proportionalto distance from the oscillator whereby the detector is energized at asimilar level for any given object Within the area,

amplifier means conectcd to the detector to amplify the signal,

temperature compensating means connected to the output of the amplifiermeans to compensate for environmental temperature variations,

a first demodulating channel connected to the compensating means andincluding a demodulator and a filter to produce a signal correspondingessentially only to the Doppler components of the intercepted signal,

a second demodulating channel including a demodulator and a filter toproduce a signal corresponding essentially to only the carrier componentof the intercepted signal, and

load means connected to the output of the channels to detect thepresence and absence of the corresponding signals in the respectivechannels.

17. The detection system of claim 16 wherein the converter includes adirect current filter in the ripple voltage supply and causing saidklystron to be periodically switched in and out of oscillation togenerate a pulsed radio frequency signal.

References Cited UNITED STATES PATENTS 2,649,538 8/1953 Marlowe et al.340-258 2,903,683 9/1959 Bagno 340-258 3,111,657 11/1963 Bagno 340-2583,149,318 9/1964 Bagno et al. 340258 3,270,339 8/1966 McEuen et al. 34353,292,096 12/1966 Deneen 33029 3,331,065 7/1967 McDonald 340258 RODNEYD. BENNETT, Primary Examiner.

C. L. WHITHAM, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,378,834 April 16, 1968 Paul I. Corbell It is certified that errorappears in the above identified patent and that said Letters Patent arehereby corrected as shown below:

Column 2, line 65, "sisignal" should read signal Column 6 line 28 "Thespeed or rate of movement of the" should read but this is compensatedfor by the fact line 31, "of" should read or Column 9, line 49, "traget"should read target Column 10, line 48, after "frequency" insert signalColumn 11, line 8, "14." should read lS. same line 8, "13" should readl4 line l4,

"l5 should read 14 (SEAL) Signed and sealed this 4th day of November196?.

Attest:

Edward M. Fletcher, Jr. WILLIAM E. SCHUYLER, JR.

Attesting Officer Commissioner of Patents

